Mechanical bar screens are frequently used in a variety of circumstances, for example, to prevent debris from entering a pump, a turbine, a water treatment device or other liquid dependent apparatus. Indeed, canals, irrigation channels and the like are frequently contaminated with so-called "natural" debris such as the limbs of trees, blocks of ice and like and with so-called "man-made" debris such as tires, bottles and the like. If such debris were permitted to enter a water dependent apparatus, such apparatus could be severely damaged or ruined.
In order to protect water dependent apparatus, it is common practice to mount a mechanical bar screen commonly referred to as a trash rack in the liquid, upstream from the apparatus to be protected. Such trash racks usually include a plurality of bars in closely spaced, side-by-side relationship relative to each other so as to define a corresponding plurality of relatively narrow and elongated flow openings. The flow openings typically have a width narrower than the width of solid material to be screened so as to allow for separation of the solid materials from the liquid flow stream.
One of the disadvantages associated with the use of conventional trash racks is that they have a tendency to become clogged with trash and other debris so as to eventually severely restrict or even terminate the flow of water to the pump or other apparatus. A recurrent cleaning of the trash rack therefore has to be performed in order to maintain proper flow to the pump, turbine or other water treating apparatus.
The prior art shows various examples of devices specifically designed for cleaning so-called trash racks. Some of these devices are particularly complex. For example, U.S. Pat. No. 4,107,040 discloses a rake member which is moved along an asymmetric S-shaped curve by a plurality of articulated arms and driven chains which are passed around a number of chain wheels to generate the asymmetric S-shaped curve. While the device may positively engage a bar screen, it is somewhat unwieldy and occupies a rather large volume. Various other rather mechanically complex trash racks are shown in the prior art. These mechanically complex trash rack cleaning devices have failed to provide satisfactory results, at least in part because of their inherent tendency towards mechanical failure and breakdown. Furthermore, their inherent somewhat prohibitive cost has rendered them unpopular. Hence, in the competitive field of trash rack cleaning devices, mechanically complex structures have failed to provide adequate results and generally have not succeeded commercially.
Two types of trash rack cleaning devices having simpler structures have succeeded in obtaining at least a partial commercial success. The first so-called "cable" type of structure is exemplified by the invention described in the Swiss patent specification No. 396,779. This type of structure includes a carriage suspended from a lift wire in abutting contact with the trash rack. The carriage is provided with teeth extending between the bars of the trash rack. A bucket is hingedly mounted to the carriage. When the carriage is allowed to drop by the action of gravitational forces, the bucket is pivoted away from the trash rack. When the carriage is pulled upwardly by the lift wire, the bucket collects the trash which has been gathered at the trash rack.
Although more reliable and less costly than the above-mentioned mechanically complex structures, the so-called "cable" type of the trash rack cleaning devices using raking components suspended by hanging wires suffer from at least two major drawbacks. First, since the wires are by nature relatively flexible and since the device relies on gravitational force for lowering the raking component, the latter may be impossible to lower when it encounters an obstacle during its descending action. Furthermore, when relatively large solid debris is present adjacent the surface of the liquid, the lack of downward pushing force may not allow the raking component to get at the debris in an attempt to move it as a whole.
A second major drawback associated with the so-called "cable" type of cleaning devices is that they are susceptible to being affected by side streams. The problem is compounded by the fact that such side streams are particularly prevalent when trash racks become obstructed.
A second type of cleaning device presenting a relatively simple structure and having achieved some commercial success is the so-called "arm" type of cleaning device. The arm-type cleaning device is exemplified by the structure disclosed in Swedish patent specification No. 302,430 and Canadian Patent No. 1,115,646.
Swedish patent specification No. 302,430 discloses a device including a rake member mounted at the distal end of a rake arm. An hydraulic mechanism is used to lower the rake adjacent the bottom of the rack with the rack member spaced from the latter. The rake member is then pressed against the trash rack before the rake arm is lifted, thus lifting the trash gathered at the trash rack towards a disposal container or chute.
Canadian Patent No. 1,115,646 discloses a trash rack cleaning device which attempts to overcome some of the above-mentioned inherent drawbacks. The structure disclosed in Canadian Patent No. 1,115,646 includes a rake member having a generally wedge-shaped body defined between wedge forming surfaces. The rake member is pivotally mounted at a distal end of the rake arm. An hydraulic mechanism is used for moving the rake arm along the trash rack and for pivoting the rake member between a scraping configuration assumed during downward movement of the rake arm and a scooping configuration assumed during the upward movement of the rake arm.
In the scraping configuration, the rake member is angled so that it engages the trash rack only with its apex, thus showing the trash gathered at the trash rack and allowing some of the smaller debris to pass between the trash rack bars. In the scooping configuration, the rake member has one of its surfaces pressed flush against the trash rack and another of its surfaces scoops the remaining trash upwardly.
Although the above-mentioned structure succeeds in allowing some of the smaller debris to flow through the trash rack bars, in situations wherein the debris is clogged together or wherein the debris is relatively large, this strategy may not always be successful and, hence, the inherent operational demands of the device are such that the device must be oversized to account for situations wherein large debris obstructs the trash rack. Indeed, it must be sized so as to be able to push on relatively large and heavy debris. Furthermore, in some situations, it may not be desirable to allow smaller debris to flow between the trash rack bars since such smaller debris may damage the downstream equipment such as turbines, pumps or the like.
As outlined above, one of the major drawbacks associated with prior art cleaning devices is that the inherent operational demands are such that they must be oversized to withstand the mechanical stresses that are seldom encountered. Thus, not only is the device oversized for most of its working cycles but a considerable amount of energy is wasted moving oversized components during the cleaning cycles. Indeed, since such devices must clean the trash rack by upwardly scooping the debris, they must access a position underneath the debris. In order to do so, they must be sized so as to be able to travel through relatively solid and large obstacles. Furthermore, in the event that the debris forms a large accumulation, the device must be sized to withstand the forces generated by the weight of the whole pile.
One of the main advantages of the present invention resided in that the nibbling capacity of the device allows the latter to divide relatively large debris into smaller debris and thus eliminates the need for oversizing the device so as to be able to clean relatively large debris that are only seldom encountered. The nibbling operation can also be performed for allowing the raking component to travel through relatively large solid obstacles during its cleaning cycle.